Non-coding RNA: Chief architects of drought-resilient roots

Root system architecture is the spatial arrangement of root components within the soil and is vital in plant productivity under drought stress conditions. Upon sensing drought, roots reprogram their transcriptome, proteome, and metabolome, which further bring about changes that help in maintaining the cellular homeostasis. Transcriptional and post-transcriptional regulations are the main regulatory processes that control this reprogramming by non-coding RNAs. Non-coding RNAs, control root growth, development, and stress responses through very complex and tightly interwoven networks consisting of various phytohormones, genes, and transcription factors. Numerous drought-inducible non-coding RNAs have been identified in plants which majorly target regulatory genes controlling osmotic stress tolerance, antioxidant defense, growth, development, delayed senescence, and phytohormone-mediated signaling during drought stress. Small non-coding RNAs such as microRNAs control many aspects of root development by regulating gene expression, while long non-coding RNAs regulate microRNAs by behaving as target mimics, quenchers, or decoys. Overexpression or silencing of drought-responsive non-coding RNAs has resulted in root system architecture modifications, signifying their importance in drought tolerance. Due to their ability to target multiple drought-responsive genes simultaneously, microRNAs maybe the most valuable candidate among all the non-coding RNAs, for developing drought-tolerant plants through genetic manipulation. This review summarizes discoveries in RNA-mediated gene silencing, to highlight the significance of long and small non-coding RNAs in sustaining plant homeostasis by modifying the root system architecture through gene expression regulation. The interactions between small RNAs, long noncoding RNAs and phytohormones that regulates primary root patterning are also discussed. Moreover, new research directions have been proposed to improve drought stress tolerance through micro-RNA mediated CRISPR-Cas modulation of root system architecture.